There exists a need for increasing the reliability of integrated, electronic circuit devices. The long term reliability of a device is often limited by the braze alloy which bonds the circuit chip of the device to its header or substrate. Thus, integrated circuits in current use frequently fail because the braze alloy fractures, causing the device to short out.
Semiconductor silicon chips are conventionally bonded to substrates with low melting point materials, including hard solders, soft solders and epoxies. Hard solders are alloys which are free from temperature fatigue failures while soft solders are subject to such failures. This distinction has restricted hard solders to the low melting gold-eutectics while soft solders include practically all lead- and tin-based alloys. Epoxies are used when the physical and chemical characteristics of the chip are adversely affected by the high temperature generated by the brazing operation. In general, when soft solders and epoxies are compared to hard solders, the soft solders and epoxies exhibit greatly inferior strength, thermal and adhesive properties. Accordingly, it has been generally concluded that hard solders give the best performance in brazing operations and best device reliability. Furthermore, the best hard solder presently available for brazing silicon dies appears to be the gold-silicon eutectic alloy. However, it has been found that even this alloy does not always give satisfactory results in that it sometimes fails to properly attach the silicon die to the substrate.
A gold-silicon eutectic alloy contains 2.85 weight percent silicon and has a minimum melting temperature of 363.degree. C. Because of thermodynamic requirements, the gold-silicon braze alloy would have to contain a greater percentage of silicon than 2.85 weight percent in order to ensure wetting of the silicon die. However, at higher than 2.85 weight percent silicon, the melting point of gold-silicon alloys increases rapidly. The high melting point of such an alloy can result in damage to the integrated circuit during the brazing operation.
It is a principal object of this invention to provide a braze alloy having a melting point that is substantially lower than the conventional gold-silicon braze alloy.
Another object of the invention is to provide a braze alloy having the ability to consistently "wet" silicon, a property of the alloy necessary for proper bonding of the die to the substrate.
A further object of the invention is to provide a braze alloy having the capability of providing a bond between the silicon die and substrate that meets established strength specifications.
Still another object of the invention is to provide a method for bonding silicon dies to a metal substrate.
Other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure.